Ariadne Conill 47f74997a7 admin guide: document use_hostname_for_dhcp.

2020-10-14 05:23:22 -06:00

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ifupdown-ng for system administrators

ifupdown-ng is a network device manager which is backwards compatible with traditional ifup and ifdown as used on Debian and Alpine systems, while solving many design deficits with the original approach through robust error handling and the use of a dependency-solver to determine interface bring-up order.

This guide is intended to walk users through using the ifupdown-ng system without any assumption of familiarity with the legacy ifupdown system.

Important Filesystem Paths

The ifupdown-ng system uses the following paths, ranked in order of importance:

/etc/network/interfaces: the interface configuration database, which contains information about what interfaces should be configured.
/etc/network/ifupdown-ng.conf: the main configuration file which controls ifupdown-ng's behaviour. See the ifupdown-ng Configuration section below.
/run/ifstate: the interface state file, which denotes what physical interfaces are configured, and what interface definition they are configured as.
/usr/libexec/ifupdown-ng: this directory contains the native ifupdown-ng executors, which are run as necessary to configure an interface. See the ifupdown-executor(7) manual page for more information on how these programs are written.
/etc/network/if-{up|down|pre-up|post-down}.d: these directories contain scripts that are run when an interface is brought up or down. In general, they follow the same contract described in ifupdown-executor(7).

All configuration examples in this guide concern the /etc/network/interfaces file.

ifupdown-ng Configuration

ifupdown-ng allows to configure some parts of it's behaviour. Currently the following settings are supported in /etc/network/ifupdown-ng.conf:

allow_addon_scripts: Enable support for /etc/if-X.d addon scripts. These are used for compatibility with legacy setups, and may be disabled for performance improvements in setups where only ifupdown-ng executors are used. Valid values are 0 and 1, default is 1.
allow_any_iface_as_template: Enable any interface to act as a template for another interface. This is presently the default, but is deprecated. An admin may choose to disable this setting in order to require inheritance from specified templates. Valid values are 0 and 1, the default is 1.
implicit_template_conversion: In some legacy configs, a template may be declared as an iface, and ifupdown-ng automatically converts those declarations to a proper template. If this setting is disabled, inheritance will continue to work against non-template interfaces without converting them to a template. Valid values are 0 and 1, the default is 1.
use_hostname_for_dhcp: A common configuration pattern with DHCP interfaces is to use hostname $(hostname). If this setting is enabled, the hostname property will default to the system hostname. Valid values are 0 and 1, the default is 1.

Interface Configuration

Basic Configuration

To begin with, lets look at a basic configuration for a desktop computer. This scenario involves using the DHCP helper to learn an IPv4 address dynamically.

In this case, the /etc/network/interfaces file would look like:

auto eth0
iface eth0
    use dhcp

These configuration statements do two things: designate that eth0 should be started automatically with the auto keyword, and designate that the dhcp executor should be used to configure the interface.

As a more detailed explanation, here is a commented version:

# Start eth0 automatically.
auto eth0

# Begin an interface definition for eth0.
iface eth0

    # Use the dhcp executor to configure eth0.
    use dhcp

IPv6 RA Configuration

With IPv6, stateless auto-configuration is typically used to configure network interfaces. If you are not interested in using IPv4 at all, you can simply use the ipv6-ra executor to ensure that an interface is configured to accept IPv6 RA advertisements:

auto eth0
iface eth0
    use ipv6-ra

Static Configuration

We can use the static executor to configure static IPv4 and IPv6 addresses. If you use the address keyword, the static executor will automatically be used to configure the interface:

auto eth0
iface eth0
    address 203.0.113.2/24
    gateway 203.0.113.1

Multiple Addresses

A typical scenario on servers is where a server has multiple IP addresses on a single interface. In this case you simply add additional address lines like this:

auto eth0
iface eth0
    address 203.0.113.2/24
    address 203.0.113.3/24
    address 203.0.113.4/24
    gateway 203.0.113.1

Dual-stack configurations

Another typical scenario for servers is to run a dual-stack configuration, where interfaces have both an IPv4 and an IPv6 address. This is accomplished in a similar way as multi-homing. You specify the IPv4 and IPv6 addresses you want, followed by gateways for each:

auto eth0
iface eth0
    address 203.0.113.2/24
    address 203.0.113.3/24
    address 203.0.113.4/24
    gateway 203.0.113.1

    address 2001:db8:1000:2::2/64
    address 2001:db8:1000:2::3/64
    address 2001:db8:1000:2::4/64
    gateway 2001:db8:1000:2::1

Relationships

As previously mentioned, ifupdown-ng features a dependency resolver that allows for determining the interface configuration order.

In order to make use of this, dependencies can be managed in one of two ways:

Explicit dependency management using `requires`

The requires keyword can be used to manage explicit dependencies:

auto eth0
iface eth0
    use dhcp

auto gre0
iface gre0
    requires eth0

    use gre
    gre-endpoint 203.0.113.2
    gre-ttl 255
    gre-flags ignore-df

    address 203.0.113.194/30
    gateway 203.0.113.193

Implicit dependency management using executors

Executors can declare implicit dependencies which work the same way as explicit dependencies, but are learned at run-time, for example:

auto bond0
iface bond0
    use bond

    bond-members eth0 eth1
    [...]

Is with respect to dependency equivalent to:

auto bond0
iface bond0
    use bond

    requires eth0 eth1
    [...]

Executors

The ifupdown-ng system is expanded with additional features via executors. Executors are selected on a per-interface basis using use statements, for example:

auto eth0
iface eth0
    use dhcp

Executors are run in the order specified by the use statements. Some executors are automatically added based on other statements in an interface definition. To see the full list of executors used for an interface, use the ifquery(8) command.

Questions

If you have further questions about how to use ifupdown-ng to configure a specific scenario, drop by the ifupdown-ng IRC channel.

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